Direct positive silver halide emulsions containing excess halide



Patented Sept. 29, 1970 3,531,290 DIRECT POSITIVE SILVER HALIDEEMULSIONS CONTAINING EXCESS HALIDE Roberta A. Litzerman, Newton, Mass.,assignor to Eastman Kodak Company, Rochester, N.Y., a corporation of NewJersey No Drawing. Filed Feb. 24, 1967, Ser. No. 618,354

Int. Cl. G03c 1/08 U.S. Cl. 96-108 29 Claims ABSTRACT OF THE DISCLOSUREDirect positive silver halide photographic emulsions, in which thehalide is at least 50 mole percent chloride, have on the surface thereofan electron acceptor or a halogen acceptor and sufficient bromide, orbromide together with iodide, to increase the speed of the silver halidegrains.

This invention relates to photographic emulsions and methods for theirpreparation. In one aspect, it relates to improved direct positivephotographic emulsions. In another aspect, it relates to improvedprocesses for preparing direct positive photographic emulsions.

Fogged direct positive photographic silver halide emulsions described inthe literature have slow speeds. It would be highly desirable to providenovel direct positive silver halide emulsions which exhibit increasedspeed, and processes for preparing such emulsions. It would also bedesirable to provide direct positive emulsions which have high maximumdensity in unexposed areas.

Direct positive photographic emulsions comprising silver halide grainsin which at least 50% of the halide is chloride are especially useful.These direct positive emulsions can be developed and processed veryrapidly. Accordingly, this invention is directed to improved directpositive photographic silver halide emulsions in which the halidecontains a substantial mole percent of chloride.

One object of this invention is to provide novel direct positivephotographic silver halide emulsions.

Another object of this invention is to provide a process for preparingnovel photographic silver halide direct positive emulsions.

A further object of this invention is to provide novel direct positivephotographic silver halide emulsions which demonstrate increased speedand high density in unexposed areas.

Still another object of this invention is to provide processes forpreparing novel direct positive photographic silver halide emulsionswhich exhibit increased speed and high density in unexposed areas.

Another object of this invention is to provide photographic elementscomprising a support having coated thereon novel silver halideemulsions.

Other objects of this invention will be apparent from the disclosureherein and the appended claims.

In accordance with one embodiment of this invention, substantiallyuniformly fogged direct positive photographic emulsions are providedwhich comprise silver halide grains, the halide of the silver halidebeing at least 50 mole percent chloride, which silver halide grains haveon the surface thereof an electron acceptor or a halogen acceptor, and asufiicient quantity of bromide to effectively increase the speed of saidsilver halide, said quantity of bromide being in addition to any bromidepresent in said grains as mixed silver halide.

In another embodiment of this invention, direct positive photographicemulsions of the type just described also have on the surface thereof asufficient quantity of iodide to effectively increase the maximumdevelopable density in the unexposed areas of the emulsion, the quantityof iodide being in addition to any iodide present in the rains as mixedsilver halide.

In still another embodiment of this invention, photographic elements areprovided which comprise a support having coated thereon a novel silverhalide emulsion of the type described herein.

In another embodiment of this invention, a process is provided forincreasing the speed of a substantially uniformly fogged, directpositive emulsion comprising silver halide grains, the halide of thesilver halide being at least 50 mole percent chloride, which includescontacting the surface of the grains with an electron acceptor or ahalogen acceptor, and a sufiicient quantity of a water soluble bromidesalt to effectively increase the speed of said silver halide grains.

In a further embodiment of this invention, a process is provided forpreparing direct positive silver halide emulsions as just described, butwhich further features contacting the silver halide grains with asufficient quantity of a water soluble iodide salt to effectivelyincrease the maximum developable density in unexposed areas of theemulsion.

In accordance with this invention, it has been found that directpositive photographic silver halide emulsions in which the halide of thesilver halide is at least 50 mole percent chloride, will exhibitsubstantially increased speed if the surface of the silver halide grainsis provided with a combination of bromide and either a halogen acceptoror an electron acceptor. It has further been found in accordance withthis invention that the maximum developable density of the emulsionsjust described can be substantially increased by the further addition ofiodide salt to the surface of said silver halide grains.

Any suitable water soluble bromide salt can be used in the practice ofthis invention. Typical useful bromide salts include the ammonium,potassium, sodium, lithium, cadmium and strontium bromide salts. Incarrying out the processes of this invention, the water soluble bromidesalt is added to the emulsion at a concentration sufficient toeffectively increase the speed of the emulsion. This concentration canbe varied over a considerable range. As a guide, it can be said that theaddition of from about .01 to about 0.2 mole bromide salt per mole ofsilver produces useful increases in speed. Preferred concentrationranges for most emulsions are in the range of about .04 to about .09mole bromide salt per mole of silver in the emulsion. Thus, potassiumbromide can be added at concentrations of from about 1 to 20 grams, andpreferably 4 to 9 grams, per mole of silver, with good results. Statedin another way, the surface of the silver halide grains of the inventioncan advantageously carry from about .01 to about 0.2 mole bromide permole of silver, and preferably from about .04 to about .09 mole bromideper mole of silver. This concentartion of bromide is, of course, inaddition to any bromide present in the emulsion as mixed silver halide.

A wide variety of water soluble iodide salts can be used in the practiceof this invention. Typical iodide salts include the ammonium, potassium,sodium, lithium, cadmium and strontium iodide salts. The quantity ofiodide added to the emulsion shauld be sufficient to effectivelyincrease the maximum developable density in the unexposed areas of theemulsion. This concentration can be varied over a wide range. As asuggested concentration, from about .002 to about .03 mole of watersoluble iodide salt, per mole of silver, can be added in the process ofthe invention to obtain effective increases in maximum developabledensity. Especially good results are obtained when the water solubleiodide salt is added in the range of from about .003 to about .012 moleper mole of silver.

For example, potassium iodide can be added at from about .5 to 5 gramsper mole of silver with good increases in maximum density. Statedanother way, the silver halide emulsions of the invention can contain,on the surface thereof, from about .002 to about .03, and preferablyfrom about .003 to about .012 mole iodide per mole of silver.

In accordance with the invention, the quantity of bromide, or bromideand iodide, present on the surface of the silver halide grains is inaddition to any bromide or iodide present in the grains as mixed silverhalide (such as that bromide or iodide present throughout the grains assilver chlorobromide, silver chloroiodide or silver chlorobromoiodide).

The electron acceptors or halogen acceptors which give particularly goodresults in the practice of this invention can be characterized in termsof their polarographic halfwave potentials, i.e., their oxidationreduction potentials determined by polarography. The electron acceptorsuseful herein have an anodic polarographic potential and a cathodicpolarographic potential which, when added to gether, give a positivesum. The halogen acceptors useful herein have an anodic polarographicpotential less than 0.85 and a cathodic polarographic potential which ismore negative than -1.0. Preferred halogen acceptors have an anodicpolarographic potential less than 0.62 and a cathodic polarographicpotential which is more negative than 1.3. Cathodic measurements can bemade with a 1 l0 molar solution of the electron acceptor in a solvent,for example, methanol which is 0.05 molar in lithium chloride using adropping mercury electrode with the polarographic halfwave potential forthe most positive cathodic Wave being designated E Anodic measurementscan be made with 1 10 molar aqueous solvent solution, for examplemethanolic solutions of the electron acceptor which are 0.05 molar insodium acetate and 0.005 molar in acetic acid using a carbon paste ofpyrolytic graphite electrode, with the voltommetric half peak potentialfor the most negative anodic response being designated E,,. In eachmeasurement, the reference electrode can be an aqueous silversilverchloride (saturated potassium chloride) electrode at 20 C.Electrochemical measurements of this type are known in the art and aredescribed in New Instrumental Methods in Electrochemistry, by Delahay,Interscience Publishers, New York, 1954; Polarography, by Kolthoff andLingane, 2nd edition, Interscience Publishers, New York, N.Y., 1952;Analytical Chemistry, 36, 2426 (1964) by Elving; and AnalyticalChemistry 30, 1576 (1958) by Adams. Signs are given according to IUPAC,Stockholm Convention 1953. Advantageously, these electron acceptors usedherein also provide spectral sensitization such that the ratio of minusblue relative speed to blue relative speed of the emulsion is greaterthan 7, and preferably greater than 10, when exposed to a tungsten lightsource through Wratten No. 16 and No. 35 plus 38A filters respectively.Such electron acceptors can be termed spec- 'trally sensitizing electronacceptors. However, electron acceptors can be used which do notspectrally sensitize the emulsion.

An especially useful class of electron acceptors which can be used inthe direct-positive photographic silver halide emulsions and processesof this invention are cyanine dyes, such as theimidazo[4,5-b]quinoxaline dyes. Dyes of this class are described inBrooker and Van Lare Belgian Pat. 660,253, issued Mar. 15, 1965. Inthese dyes, the imidazo[4,5-b]quinoxaline nucleus is attached, throughthe Z-carbon atom thereof, to the methine chain.

Very good results are obtained with 2-aromatically substituted indoledyes, e.g., cyanine dyes containing an indole nucleus aromaticallysubstituted in the 2-position, i.e., a cyanine dye containing a2-aromatically substituted indole nucleus. Advantageously, such dyesalso include a desensitizing nucleus in addition to the indole nucleus.The desensitizing nucleus is one which, when converted to a symmetricalcarbocyanine dye and added to a silver chlorobromide emulsion containing40 mole percent chloride and 60 mole percent bromide, at a concentrationin the range of about 0.01 to about 0.2 g. of dye per mole of silver,causes, by electron trapping, at least an loss in speed to blueradiation, and preferably more than a or loss in blue speed. One usefulclass of spectral sensitizing electron acceptors suitable for use inthis invention has the following general formula:

where L represents a methane chain containing from 2 to 3 carbon atoms;A represents a 2-aromatically substituted indole nucleus attached to themethine chain through the 3-carbon atom of the indole nucleus; and Brepresents an organic heterocyclic nucleus, said nucleus being, where Lrepresents a methine chain of 2 carbon atoms, a desensitizing nucleus toprovide an unsymmetrical dimethine cyanine dye, and, where L representsa methine chain of 3 carbon atoms, B represents a Z-aromaticallysubstituted indole nucleus attached to the methine chain through the3-carbon atom of the indole nucleus. An especially useful desensitizingnucleus, where L is a methine chain containing 2 carbon atoms, is animidazo [4,5-b]quinoxaline nucleus attached through the 2-carbon atomthereof to the methine chain. Spectral sensitizing electron acceptors ofthis type are dyes and can be prepared using any of the methodsgenerally used for preparing such dyes. One convenient method involvesrefluxing, in a suitable solvent, a carboxaldehyde derivative of a2-aromatically substituted indole with an alkyl substituted quaternarysalt of a compound containing the desired desensitizing nucleus. Forexample, a 2-aromatically substituted indol-3-carboxaldehyde can berefluxed in a solvent such as acetic anhydride with a2-alkylimidazo[4,5-b]quinoxalinium salt or a 2-alkylene pyrrolo[2,3-b1pyridine compound to provide the desired dye.

A preferred group of spectral sensitizing electron acceptors employedherein has the following general formula:

wherein L represents a methine linkage, e.g., -CH=, C(CH -C(C,-,H etc.;A represents an aromatic nucleus, such as a phenyl nucleus which cancontain various groups, such as alkyl (e.g., methyl, ethyl, propyl,butyl, etc.), alkoxy (e.g., methoxy, ethoxy, propoxy, butoxy, etc.),halogen groups such as Br, C1 or F, aryl such as phenyl, or A can be aheterocyclic aromatic nucleus, preferably containing from 5 to 6 carbonatoms, and the hetero atom is preferably nitrogen, sulfur or oxygen; Rand R each represents a hydrogen atom, a halogen atom such as Cl, Br orF, an alkyl or alkoxy substituent such as methyl, ethyl, propyl, butyl,methoxy, propoxy, hydroxy ethyl, etc.; or, R and R taken together,represent the atoms necessary to complete a fused aromatic ring having 6carbon atoms; R represents an alkyl substituent (including substitutedalkyl) preferably containing from 1 to 8 carbon atoms, including methyl,ethyl, propyl, butyl, octyl, sulfoalkyl such as sulfopropyl orsulfobutyl, sulfatoalky such as sulfatopropyl or sulfatobutyl,carboxyalkyl such as carboxyethyl or carboxybutyl and the like; R and Reach represents an alkyl substituent (including substituted alkyl),preferably containing from 1 to 18 carbon atoms, including methyl,ethyl, propyl, isopropyl, butyl, sec-butyl, hexyl, dodecyl, octadecyl,benzyl, beta-phenylethyl, etc., sulfoalkyl such as sulfatobutyl;carboxyalkyl such as carboxyethyl and carboxybutyl; allyl; alkenyl suchas propenyl and butenyl; alkynyl such as propargyl; cycloalkyl such ascyclobutyl and cyclohexyl; dialkylaminoalkyl such as dimethylaminoethyland aryl such as phenyl, p-tolyl, o-tolyl, 3,4- dichlorophenyl, etc.; Rhas the same meaning as R or, taken together with R represents analkylene group such as trimethine or dimethine; R represents halogen, CNor N n is an integer from 0 to 3, and, X represents an anion, preferablyan acid anion such as chloride, bromide, iodide, p-toluenesulfonate,thiocyanate, sulfonate, methyl sulfate, ethyl sulfate, perchlorate, etc.

A related class of highly useful spectral sensitizing electron acceptorsare pyrrolo[2,3-b]pyrido dyes, e.g., those having the following formula:

wherein R R and R each represents an alkyl group, such as methyl, ethyl,propyl or butyl, or an aryl group such as phenyl; L and X have themeaning given above; and Q represents a substituent selected from thegroup consisting of (1) --L=Q wherein Q represents the atoms necessaryto complete a desensitizing nucleus to form a trimethine cyanine dye,such as a 6-nitrobenzthiazole nucleus, a S-nitroindolenine nucleus, animidazo [4,5-b]quinoxaline nucleus or a pyrrolo[2,3-b]pyrido nucleus,e.g.,

Rm R11 C \N N wherein R R and R each represents the same groups givenfor R R and R and (2) the atoms necessary to complete a desensitizingnucleus to form a dimethine cyanine dye, such as a pyrazole nucleus or a2-aromatically substituted indole nucleus which is attached, through the3-carbon atom thereof to the methine chain, e.g.,

C R3 11% in it wherein R R R and R have the same meanings given above.

Another useful group of spectral sensitizing electron acceptors have thefollowing general formula:

Formula II R R9 X Ru R1 6 using the method described in Coenen et al.U.S. Pat. 2,930,694, issued Mar. 29, 1960.

Symmetrical imidazo[4,5-b]quinoxaline trimethine cyanine dyes, whereineach nucleus is attached through the Z-carbon atom thereof to themethine chain, are useful electron acceptors in the practice of thisinvention. Typical of such dyes are those having the following generalformula:

wherein X, L, R, and R have the meanings given above, and R and R havethe same values given for R and R each X is halogen such as Br, Cl and Fand each n is an integer from 0 to 3. Dyes of this type can be preparedby the method described in Belgian Pat. 660,253, published Mar. 15,1965.

Still another group of electron acceptors are pyrazolyl dyes, such asthose having the following general formula:

wherein R 11, R R L and X each have the meanings given in Formula Iabove, R and R each represents a substituent selected from the groupconsisting of hydrogen atom, an alkyl substituent, preferably containing1 to 18 carbon atoms, as exemplified by methyl, butyl, octyl, dodecyl,octadecyl, an aryl substituent such as phenyl, p-tolyl,3,4-dichlorophenyl, etc., and R has the same value as R Dyes of thistype can be conveniently prepared by conventional techniques suitablefor preparing such material. For example, a suitable method involvesrefluxing in a suitable solvent such as acetic anhydride, a2-alkylimidazo[4,5-b]quinoxalinium salt with a pyrazole4-carboxaldehdye. A typical dye of this type is 1,3-dially1-2- [2- 3 ,5-dimethyl- 1-phenyl-4-pyrzolyl) vinyl] imidazo[4,5 b1quinoxaliniumiodide which has the formula:

This dye can be prepared by refluxing 1,3-diallyl-2- methylimidazo[4,S-b]quinoxalinium p -toluenesu1fonate with3,5-dimethyl-l-phenyl-pyrazole 4-carboxyaldehyde in acetic anhydride fora suitable time, e.g., 10 minutes.

Still another class of useful spectral sensitizin electron acceptors arenitro-substituted dyes, such as the cyanine and merocyanine dyes inwhich at least one nucleus, and preferably two nuclei, containdesensitizing substituents such as N0 Some other specific electronacceptors which give outstanding results in the practice of thisinvention are the reaction product of a cyanine dye with a halogenatingagent. Preferred electron acceptors of this type are those wherein ahydrogen atom of at least one methine group of the cyanine dye isreplaced with a halogen atom having an atomic weight in the range ofabout 7 35 to about 127, i.e., chlorine, bromine or iodine atoms. Inthese compounds, one carbon atom linking the two nuclei thereof cancarry two halogen atoms. Suitable halogen containing compounds can berepresented by zole, S-chlorobenzothiazole, 7-chlorobenzothiazole, 4-methylbenzothiazole, S-methylbenzothiazole, S-bromobenzothiazole,4-phenylbenzothiazole, S-phenylbenzothiazole, 6-phenylbenzothiazole,4-methoxybenzothiazole, S-methoxybenzothiazole, S-iodobenzothiazole,4-ethoxyr benzothiazole, S-ethoxybenzothiazole, S-hydroxybenzothiazole,etc.); the naphthothiazole series (e.g., u-naphthothiazole,,B-naphthothiazole, S-rnethoxy-B-naphthothiazole, ethyl pnaphthothiazole, 8 methoxy anaphthothiazole,7-methoxy-a-naphthothiazole, etc.); those of the benzoxazole series(e.g,, benzoxazole, 5-chlorobenzoxazole, S-methylbenzoxazole,S-phenylbenzoxazole, S-methoxybenzoxazole, S-ethoxybenzoxazole,S-hydroxybenzoxazole, etc.); those of the naphthoxazole series (e.g.,u-naphthoxazole, ,B-naphthoxazole, etc.); those of r the benzoselenazoleseries (e.g., benzoselenazole, 5- chlorobenzoselenazole,S-methylbenzoselenazole, S-hydroxybenzoselenazole, etc.); those of thenaphthoselenazole series (e.g., a-naphthoselenazole,fl-naphthoselenazole, etc.); those of the quinoline series including the2- 0 quinolines (e.g., quinoline, 3-methylquinoline, S-methylquinoline,7-methylquinoline, S-methylquinoline, 6-choroquinoline,8-chloroquinoline, 6-methoxyquinoline, 6- hydroxyquinoline,84hydroxyquinoline, etc.); the 4-quinolines (e.g., quinoline,6-methoxyquinoline, 7-methoxyquinoline, S-methoxyquinoline, etc.); thoseof the isoquinoline series (e.g., the l-isoquinolines, the3-isoquinolines, etc.); each L represents a methine linkage as describedabove; X represents a chlorine, bromine or iodine atom; X and X eachrepresents an atom selected from the group consisting of hydrogen,chlorine, bromine and iodide, at least one of X and X being chlorine,bromine or iodine; R and R each represents alkyl, e.g., lower alkyl suchas methyl, ethyl, propyl, isopropyl, butyl, secondary butyl, tertiarybutyl, etc., a sulfo-alkyl group in which the alkyl group has from 1 to4 carbon atoms, such as sulfomethyl, sulfoethyl, sulfopropyl,sulfobutyl, etc., and a carboxyalkyl group in which the alkyl group hasfrom 1 to 4 carbon atoms such as carboxymethyl, carboxyethyl,carboxypropyl, carboxybutyl, etc.; A represents an acid anion such aschloride, bromide, iodide, ptoluenesulfonate, thiocyanate, methylsulfate, ethyl sulfate, perchlorate, and the like; y represents aninteger of from: 1 to 3 and d, m, n, and p each represents a positiveinteger of from 1 to 2.

The halogen containing compounds described above can be prepared byhalogenating a cyanine dye with chlorine, bromine or iodine. Anysuitable halogenating agent may be used, such as aqueous alcoholic(e.g., methanol or ethanol) solutions of the halogen, N-chloro'suecinimide, N-bromosuccinimide, N-iodosuccinimide, or a commerciallyavailable halogen-pyrrolidone complex, such as a bromo-pyrrolidonecomplex sold by General Aniline and Film Corp. Using such halogenatingagents causes replacement by halogen of a hydrogen atom in the methinechain. In carbocyanines, or dicar- 8 bocyanines, it is believed thathalogen substitution occurs on a terminal carbon atom of the methinechain. As noted above, one linking carbon atom can carry two halogenatoms.

The compounds which accept electrons in the direct positive photographicsilver halide emulsions and processes of this invention can be employedin widely varying concentrations. However, such compounds are preferablyemployed at concentrations in the range of about milligrams to about 2grams of electron acceptor per mole of silver halide. Best results areobtained using from 300 to 600 milligrams electron acceptor per mole ofsilver halide. Specific examples of suitable electron acceptors include;

1,1-dimethy1-2,2-diphenyl-3,3 -indolocarbocyanine bromide;

2,2-di-p-methoxyphe-nyll, l-dimethyl3,3 '-indlo carbocyanine bromide;

l,1'-dimethyl-2,2',8-triphenyl-3,3'-indolocarbocyanine perchlorate;

1,3-diallyl-2- [2- 9-methyl-3 -carbazolyl vinyl] -imidazo [4,5 -b]quinoxalinium p-toluenesulfonate;

1,3-diethyl-l-methyl-2-phenyl imidazo [4,5-b]quinoxalino-3'-indo1ocarboeyanine iodide;

1,1',3,3 '-tetraethylimidazo [4,5 -b] quinoXalinocarbocyanine chloride;

6-chlo ro- 1 -methyl- 1 ,2',3-triphenylimidazo [4,5 -b]quinoxalinocarbocyanine chloride;

6-chloro- 1 -methyl- 1 ,2,3-triphenylimidazo [4,5 -b]quinoxalino-3-indolocarbocyanine p-toluenesulfonate;

6,6'-dichloro-l,13,3 -tetraphenylimidazo [4,5 -b quinoxalinocarbocyaninep-toluenesulfonte;

l, 1 3 ,3 -tetramethyl-2-phenyl-3 -indolopyrrolo [2,3-b]

pyridocarbocyanine iodide;

1,1',3 ,3, 3 ,3-hexamethylpyrro1o [2,3 -b pyridocarbocyanineperchlorate;

I,1',3,3-tetramethyl-5-nitro-2'-phenylindo-3 indolocarbocyanine iodide;

1,1',3,3,3,3-hexamethyl-5,5'-dinitroindocarbocyanine p-toluenesulfonate;

3'-ethyl-1=methyl-2-phenyl-6'-nitro-3-indolothia carbocyanine iodide;

5-chloro-1,3-dimethyl-2-phenyl-6-nitro-3-indolocarbocyaninep-toluenesulfonate;

5,5'-dichloro-3,3-diethyl-6,6-dinitrothiacarbocyanine iodide;

pinacryptol yellow,

5-m-nitrobenzylidenerhodanine,

5-m-nitrobenzylidene-3:phenylrhodanine,

1,3-diallyl-2-[2-(3,5-dimethyl-1-phenyl-4-pyrazolyl) I vinyl] imidazo[4,5 -b] quinoxalinium iodide,

3-ethyl-5-m-nitrobenzylidenerhodanine,

3-ethyl-5-(2,4-dinitrobenzylidene)rhodanine,

S-o-nitrobenzylhlene-3-phenylrhodanine,

1,3-diethyl-6-nitrothia-2'-cyanine iodide,

6-chloro-4-nitro-benzotriazole,

6-amino-1-'methyl-2-[1'-(methyl-6'-quinolinium)vinyl] quinoliniumdichloride;

4- p-n-amyloxyplienyl -2,6-di p-ethylphenyl) thiapyrylium perchlorateand the like.

one of the following formulas: 5

wherein Z and Z each represents the non-metallic atoms necessary tocomplete a heterocyclic nucleus of the type used in cyanine dyes, suchas a nucleus of the benzothiazole series (e.g., benzothiazole,4-chlorobenzothia- 20 .A preferred class of halogen acceptors that canbe used in the practice of this invention comprises the spectralsensitizing merocyanine dyes having the formula:

and n is an integer from to 2, i.e., 0, 1 or 2.

Halogen accepting merocyanine dyes which can be employed in the practiceof this invention can also be represented by the formula:

where lm represents an integer of from 1 to 3, R represents an alkylgroup (including substituted alkyl) and preferably containing from 1 to8 carbon atoms, such as methyl, ethyl, propyl, butyl, octyl, sulfoalkylsuch as sulfopropyl or sulfobutyl, sulftoalkyl such as sulfatopropyl 0rsulfatobutyl, or carboxyalkyl such as carboxyethyl or carboxybutyl, oran aryl group (including substituted aryl), e.g., phenyl, sulfophenyl,carboxyphenyl, tolyl and the like, each L represents a methine group,substituted or unsubstituted, n is a positive integer from 1 to 3, Zrepresents the non-metallic atoms necessary to complete a heterocyclicnucleus containing from to 6 atoms in the heterocyclic ring, e.g., anucleus of the benzothiazole series, a nucleus of the benzoxazoleseries, a nucleus of the benzoselenazole series, a nucleus of thea-naphtholthiazole series, a nucleus of the B- naphthothiazole series, anucleus of the a-naphthoxazole series, a nucleus of the ,G-naphthoxazoleseries, a nucleus of the a-napthoselenazole series, a nucleus of the B-naphthoselenazole series, a nucleus of the thiazoline series, a nucleusof the simple thiazole series (e.g., 4- methylthiazole,4-phenylthiazole, 4-(2-thienyl)-thiazole, etc.), a nucleus of the simpleselena-zole series (e.g., 4- methylselenazole, 4-phenylselenazole, etc.)a nucleus of the simple oxazole series (e.g., 4-methyloxazole, 4-phenylthiazole, etc.) a nucleus of the quinoline series, a nucleus ofthe pyridine series, a nucleus of the 3,3- dialkylindolenine and thelike, and Q represents the nonmetallic atoms necessary to complete aheterocyclic nucleus containing 5 atoms in the heterocyclic ring, e.g.,a rhodanine nucleus, a 2-thio-2,4(3,5)-oxazoledione nu cleus, a2-thiohydantoin nucleus, a S-pyrazolone nucleus, etc.

A more preferred class of halogen accepting compounds which can beemployed in the practice of this invention can be represented by theformula:

where each R represents an alkyl group (including substituted alkyl) andpreferably containing from 1 to 8 carbon atoms, such as methyl, ethyl,propyl, butyl, octyl, sulfoalkyl such as sulfopropyl or sulfobutyl,sulfatoalkyl such as sulfatopropyl or sulfatobutyl, or carboxyalkyl suchas carboxyethyl or carboxybutyl, or an aryl group (including substitutedaryl), e.g., phenyl, sulfophenyl, carboxyphenyl, tolyl and the like, Itis a positive integer from 1 to 2, Z represents the nonmetallic atomsnecessary to complete a heterocyclic nucleus containing from 5 to 6atoms in the heterocyclic ring, as defined in the previous formula, andX represents an oxygen atom, a sulfur atom, a selenium atom or a groupof the formula where R represents an alkyl group (including substitutedalkyl) and preferably containing from 1 to 8 carbon atoms, such asmethyl, ethyl, propyl, butyl, octyl, sulfo- 10 alkyl such as sulfopropylor sulfobutyl, sulfatoalkyl such as sulfatopropyl or sulfatobutyl, orcarboxyalkyl such as carboxyethyl or carboxybutyl, or an aryl group(including substituted aryl), e.g., phenyl, sulfophenyl, carboxyphenyl,tolyl and the like. Suitable procedures for preparing dyes employed inthe practice of this invention are described in Brooker et al. US. Pats.2,493,747 and 2,493,748, issued I an. 10, 1950.

Specific examples of halogen accepting compounds which can be employedin the practice of this invention include:

3 -carb oxymethyl-S- 3 -ethyl-2-b enzothiazolinylidene) ethylidene]rhodanine;

3-ethyl-5- 3-ethyl-2-benzothiazolinylidene) ethylidene1rhodanine;

3- Z-dimethylaminoethyl) -5- [4- 3-ethyl-2-benzothiazolinylidene-2-butenylidene] rhodanine;

3-ethyl-5- (3-ethyl-2-benzoxazolinylidene ethylidene] rhodanine;

3 -carb oxymethyl-S- 3 -ethyl-2-b enzoxazolinylidene ethylidene]rhodanine;

3 -carb oxymethyl-S (3-methyl-2-thiazolidinylidene l-methylethylidene]rhodanine 3 -carb oxymethyl-5-( 3 -ethyl-4-methyl-4-thiazolin-2-ylidene) rhodanine 5- 3-methyl-2-thiazolidinylidene -1-methylethylidene]-3 (2-sulfoethyl rhodanine;

3-ethyl-5-[1-(4-sulfobutyl)-4( 1H) -pyridylidene] rhodanine sodium salt;

3-ethyl-5-( 1-ethyl-4( 1H) -pyridylidene) rhodanine;

3-ethyl-5 3 -ethyl-Z-benzothiazolinylidene) ethylidene]-2-thio-2,4-oxazolidenedione;

3-carb oxymethyl-S- 3 -ethyl-2-benzoxazolinylidene) ethylidene]-2-thio-2,4-oxazolidinedione;

3-ethyl-5- (3-ethylnaphth [2, l-d] oxazolin-Z-ylidene) ethylidene]-2-thio-2,4-oxazolidinedione;

l-carboxymethyl-S 3-ethyl-2-benzothiazolinylidene ethylidene] -3-phenyl-2-thiohydantoin;

1-carboxymethyl-5-[ (e-ethyl-Z-b enzoxazolinylidene ethylidene]-3-phenyl-2-thiohydantoin;

1-carboXymethyl-5-[ l-ethylnaphtho 1,2-d] thiazolin- 2-ylideneethylidene] -3 -phenyl-2-thiohydantoin;

3 -heptyl-5- 1-methylnaphtho[ 1,2-d] thiazolin-Z- ylidene-1-phenyl-2-thiohydantoin;

5- [4- 3 -ethyl-2-benzoxazolinylidene -2-butenylidene] 1, 3-diphenyl-2-thiohyd antoin;

4-[ l-ethylnaphtho 1,2-d] thiazolin-Z-ylidene) methylethylidene]-3-methyl- 1- (4-sulfophenyl) -2- pyrazolin-S-one;

l-ethoxycarbonylmethyl-S-[ 1-ethylnaphtho[1,2-d]- thiazolin-Z-ylideneethylidene] -3 (4-nitrophenyl -2-thiohydantoin;

5- [4- 3-ethyl-2-benzothiazolinylidene -2-butenylidene] 3-heptyl-2-thio-2,4-oxazolidinedione;

5- 1,3-diallylimidazo [4,5 -b] quinoxalin-Z 3H ylidene ethylidene] 3-ethylrhodanine;

3-ethyl-5- 3-methyl-2-thiazolinylidene ethylidene]2-thio-2,4-oxazolidinedione;

5- (3- Z-carboxyethyl) -2-thiazolinylidene -ethylidene] 3-etl1yl-rhodanine;

5- 3 -methyl-2-thiazolidinylid ene) -1-methylethylidene] -3-2-morpholinoethyl rhodanine;

5-[ (3- (2-carboXyethyl-2-thiazolidinylidene) -1-methylethylidene] -3-carb oxymethylrhodanine;

5 (3 (Z-carboxyethyl) -2-thiazolidinylidene) -1- methylethylidene] -3(Z-methoxyethyl rhodanine;

3- 3-dimethyl aminopropyl -5- (3-methyl-2-thiazolidinylidene)ethylidene] rhodanine.

The halogen accepting compounds employed in the practice of thisinvention can be used in widely varying concentrations. However, thehalogen accepting compounds are generally employed at concentrations inthe range of about 200 mg. to about 1.0 g., preferably about 300 toabout 600 milligrams per mole of silver halide.

If desired, the emulsions of the invention can be provided with acombination of electron acceptor and halogen acceptor.

In carrying out the process of this invention, we have found that highlyuseful increases in speed can be achieved when the water soluble bromidesalt is added to the emulsion prior to the addition of the electronacceptor or halogen acceptor. However, the most pronounced increases inspeed are achieved when the halogen acceptor or electron acceptor isadded to the emulsion prior to the addition of the water soluble bromidesalt. The increased speed obtained by this latter preferred sequence ofaddition does not require a holding period between the addi tion ofhalogen or electron acceptor and salt. Good speed increases are obtainedby stirring the emulsion, adding halogen or electron acceptor and thenadding the salt with continued stirring but without a holding period.However, a holding period can be used.

The direct positive silver halide emulsions useful herein can beuniformly fogged in any suitable manner, such as by light or withchemical fogging agents. Chemical fogging agents are preferred. Typicaluseful chemical fogging agents include reducing agents such as stannouschloride, formaldehyde, thiourea dioxide and the like. In preferredembodiments of this invention, the emulsion is fogged by the additionthereto of a reducing agent, such as thiourea dioxide, and a compound ofa metal more electropositive than silver, such as a gold salt (e.g.,potassium chloroaurate) as described in British Pat. 723,019 (1955).

Typical reducing agents that are useful in providing such emulsionsinclude stannous salts, e.g., stannous chloride, hydrazine, sulfurcompounds such as thiourea dioxide, phosphonium salts such astetra(hydroxymethyl) phosphonium chloride, and the like. Typical usefulmetal compounds that are more electropositive than silver include gold,rhodium, platinum, paladium, iridium, etc., preferably in the form ofsoluble salts thereof, e.g., potassium chloroauratc, auric chloride, (NHPdCl and the like.

Useful concentrations of reducing agent and metal compound (e.g., metalsalt) can be varied over a considerable range. As a general guideline,good results are obtained using about .05 to 40 mg. reducing agent permole of silver halide, and 0.5 to 15.0 mg. metal compound per mole ofsilver halide. Best results are obtained at lower concentration levelsof both reducing agent and metal compound.

As used herein, and in the appended claims, fogged refers to emulsionscontaining silver halide grains which produce a density of at least 0.5when developed, without exposure, for 5 minutes at 68 F. in developerKodak DK-SO having the composition set forth below, when the emulsion iscoated at a silver coverage of 50 mg. to

500 mg. per square foot.

DEVELOPER G. N-methyl-p-aminophenol sulfate 2.5 Sodium sulfite(anhydrous) 30.0 Hydroquinone 2.5 Sodium metaborate 10.0 Potassiumbromide 0.5

Water to make 1.0 1.

This invention can be practiced with direct positive emulsions of thetype in which a silver halide grain has a water-insoluble silver saltcenter and an outer shell composed of a fogged water-insoluble silversalt that develops to silver without exposure. These emulsions can beprepared in various ways, such as those described in Berriman US. patentapplication Ser. No. 448,467, filed Apr. 15, 1965, now US. Pat.3,367,778, isued Feb. 6, 1968. For example, the shell of the grains insuch emulsions may be prepared by precipitating over the core grains alightsensitive water-insoluble silver salt that can be fogged and whichfog is removable by bleaching. The shell. is of sufiicient thicknes toprevent access of the developer used in processing the emulsions of theinvention to the core. The silver salt shell is surface fogged to makeit developable to metallic silver with conventional surface imagedeveloping compositions. The silver salt of the shell is sufiicientlyfogged to produce a density of at least about 0.5 when developed for 6minutes at 68 F. in Developer A below when the emulsion is coated at asilver coverage of mg. per square foot. Such fogging can be effected bychemically sensitizing to fog with the sensitizing agents described forchemically sensitizing the core emulsion, high intensity light and thelike fogging means well known to those skilled in the art. While thecore need not be sensitized to fog, the shell is fogged. Fogging bymeans of a reduction sensitizer, a noble metal salt such as gold saltplus a reduction sensitizer, a sulfur sensitizer, high pH and low pAgsilver halide precipitating conditions, and the like can be suitablyutilized. The shell portion of the subject grains can also be coatedprior to fogging.

DEVELOPER A N-methyl-p-aminophenol sulfate-2.5 g. Ascorbic acid-10.0 g.Potassium metaborate-35 .0 g. Potassium bromide1.0 g. Water to 1 literpH of9.6

Before the shell of water-insoluble silver salt is added to the silversalt core, the core emulsion is first chemically or physicaly treated bymethods previously described in the prior art to produce centers whichpromote the deposition of photolytic silver, i.e., latent imagenucleating centers. Such centers can be obtained by various techniquesas described in the Berriman application referred to above. Silver saltcores containing centers attributable to a metal of Group VIII of thePeriodic Table, e.g., palladium, iridium or platinum and the like, areespecially useful since these centers also appear to function aselectron acceptors. Chemical sensitization techniques of the typedescribed by Antoine Hautot and Henri Saubeneir in Science et IndustriesPhotographiques, vol. XXVIII, January 1957, pages 1 to 23 and January1957, pages 57 to 65 are particularly useful. Such chemicalsensitization includes three major classes, namely, gold or noble metalsensitization, sulfur sensitization, such as by a labile sulfurcompound, and reduction sensitization, e.g., treatment of the silverhalide with a strong reducing agent which introduces small specks ofmetallic silver into the silver salt crystal or grain.

The practice of this invention is particularly suitable for high speeddirect positive emulsions comprising fogged silver halide grains and acompound which accepts electrons, as described and claimed inlllingsworth US. patent application Ser. No. 609,794, filed Jan. 17,1967, now abandoned and titled Photographic Reversal Materials III. Thefogged silver halide grains of such emulsions are such that a testportion thereof, when coated as a photographic silver halide emulsion ona support to give a maximum density of at least about one uponprocessing for six minutes at about 68 F. in Kodak DK-SO developer, hasa maximum density which it at least about 30% greater than the maximumdensity of an identical coated test portion which is processed for sixminutes at about 68 F. in Kodak DK5O developer after being bleached forabout 10 minutes at about 68 F. in a bleach composition of:

Potassium cyanide50 irng. Acetic acid (glacial)3 .47 cc. Sodium acetate11.49 g. Potassium bromide1 19 mg. Water to 1 liter The grains of suchemulsions will lose 'at least about 25% and generally at least about 40%of their fog when bleached for ten minutes at 68 F. in a potassiumcyanide bleach composition as described herein. This fog loss can beillustrated by coating the silver halide grains as a photographic silverhalide emulsion on a support to give a maximum density of at least 1.0upon processing for six minutes at about 68 F. in Kodak DK-50 developerand comparing the density of such a coating with an identical coatingwhich is processed for six minutes at 68 F. in Kodak DK-SO developerafter being bleached for about minutes at 68 F. in the potassium cyanidebleach composition. As already indicated, the maximum density of theunbleached coating will be at least 30% greater, generally at least 60%greater, than the maximum density of the bleached coating.

The silver halides employed in the preparation of the photographicemulsions useful in this invention include any of the photographicsilver halides which contain at least 50 mole percent chloride, asexemplified by silver chloride, silver chlorobromide, silverchlorobromoiodide, and the like. Emulsion blends, e.g., blends of silverchloride and silver chlorobromide, can be used. Also, the core of thesilver halide grain can be composed of silver halide of differentcomposition than that in the outer shell of the grain. In any case, thetotal chloride present as silver chloride or silver chlorohalide shouldbe at least 50 mole percent of the total halide in the emulsion.

Silver halide grains having an average grain size less than about onemicron, preferably less than about 0.5 micron, give particularly goodresults. The silver halide grains can be regular and can be any suitableshape such as cubic or octhedral, as described and claimed inIllingsworth U.S. patent application Ser. No. 609,778, filed J an. 17,1967, now abandoned and titled Direct Positive Photographic Emulsions I.Such grains advantageously have a rather uniform diameter frequencydistribution, as described and claimed in Illingsworth U.S. patentapplication Ser. No. 609,790, filed Jan. 17, 1967, now abandoned titledPhotographic Reversal Emulsions II. For example, at least 95%, byweight, of the photographic silver halide grains can have a diameterwhich is within about 40%, preferably within about 30% of the mean graindiameter. Mean grain diameter, i.e., average grain size, can bedetermined using conventional methods, e.g., as shown in an article byTrivelli and Smith entitled Empirical Relations Between Sensitometricand Size-Frequency Characteristics in Photographic Emulsion Series inThe Photographic Journal, vol. LXXIX, 1949, pages 330- 338. The foggedsilver halide grains in these direct-positive photographic emulsions ofthis invention produce a density of at least 0.5 when developed Withoutexposure for five minutes at 68 F. in Kodak DK-50 developer when such anemulsion is coated at a coverage of 50 to about 500 mg. of silver persquare foot of support. The photographic silver halides can be coated atsilver coverages in the range of about 50 to 500 milligrams of silverper square foot.

In the preparation of the above photographic emulsions, the electronacceptors, halogen acceptor, bromide and iodide salts are advantageouslyincorporated in the washed, finished silver halide emulsion and should,of course, be uniformly distributed throughout the emulsion. The methodsof incorporating such addenda in emulsions are relatively simple andwell known to those skilled in the art of emulsion making. For example,it is convenient to add them from solutions in appropriate solvents, inwhich case the solvent selected should be completely free from anydeleterious effect on the ultimate light-sensitive materials. Methanol,isopropanol, pyridine, water, etc., alone or in admixtures, have provensatisfactory as solvents for the electron acceptors and halogenacceptors. The type of silver halide emulsions that can be sensitizedwith these dyes include any of those prepared with hydrophilic colloidsthat are known to be satisfactory for dispersing silver halides, forexample, emulsions comprising natural materialssuch as gelatin, albumin,agaragar, gum arabic, alginic acid, etc., and hydrophilic syntheticresins such as polyvinyl alcohol, polyvinyl pyrrolidone, celluloseethers, partially hydrolyzed cellulose acetate, and the like. Thebinding agents for the emulsion layer of the photographic element canalso contain dispersed polymerized vinyl compounds. Such compounds aredisclosed, for example, in US. Pats. 3,142,568; 3,193,386; 3,062,674 and3,220,844 and includes the water insoluble polymers of alkyl acrylatesand methacrylates, acrylic acid, sulfoalkyl acrylates or methacrylatesand the like.

The novel emulsions of this invention may be coated on any suitablephotographic support, such as glass, film base such as celluloseacetate, cellulose acetate butyrate, polyesters such as polyethyleneterephthalate, paper, baryta coated paper, polyolefin coated paper,e.g., polyethylene or polypropylene coated paper, which may be electronbombarded to promote emulsion adhesion, to produce the novelphotographic elements of the invention.

This invention will be further illustrated by the following examples. Inthese examples, the bromide and iodide salts are added subsequent to thehalogen acceptor or electron acceptor, unless otherwise indicated.

Example 1 shows the substantial improvement in speed achieved inaccordance with the invention when soluble bromide salts are added to afogged direct positive emulsion in which the halide of the silver halideis at least 50% chloride, and which silver halide contains a halogenacceptor or an electron acceptor. The example also shows the improvementin maximum density obtained when iodide salts are employed incombination with bromide salts. This example further demonstrated thatthe use of bromide salt alone, i.e., in the absence of a halogenacceptor or an electron acceptor, fails to provide speed increases.

Example 1 A gelatin silver chloride having an average grain size ofabout 0.3 micron is prepared by adding ana queous solution of potassiumchloride and an aqueous solution of silver nitrate, simultaneously, to arapidly agitated aqueous gelatin solution at a temperature of C., over aperiod of about 35 minutes. The emulsion is chill-set, shredded andwashed by leaching with cold water in the conventional manner. Theemulsion is reduction-gold fogged by first adding 0.2 mg. of thioureadioxide per mole of silver and heating for 60 minutes at 65 C. and thenadding 4.0 mg. of potassium chloroaurate per mole of silver and heatingfor 60 minutes at 65 C. The emulsion is divided into several portions.Halogen acceptors, electron acceptors, potassium bromide and potassiumiodide are added to various portions as shown in Table I. The emulsionsobtained, along with control emulsions, are coated on cellulose acetatefilm supports at mg. silver per square foot and 350 mg. gelatin persquare foot, chill set and dried. The coatings are exposed on anintensity scale sensitometer, developed for three minutes at 65 C. inKodak developer D-19, fixed, washed and dried with the followingresults. Speeds are read at 0.3 below Dmax. in all examples.

TABLE 1 Halogen or electron acceptor (g./rnole Ag) KBr g./ Relative moleAg clear speed Example 2 demonstrates the preferred order of addi tionof halogen or electron acceptors prior to addition of the solublebromide salt (or bromide and iodide salts). This order of additionresults in even greater increases in speed than is achieved when thebromide salt (or bromide and iodide salts) are added to the emulsionprior to the addition of electron or halogen acceptors.

Example 2 An emulsion is prepared as in Example 1 except that the grainsare allowed to grow to have an average size of about 0.7 micron. Theemulsion is divided into several portions. Halogen or electron acceptorsare added to portions of the emulsion both before and after the additionof the salts, with emulsion stirring, but no holding period, between theadditions. The type and order of addition is indicated in Table II.These emulsions, along with controls, are coated, exposed and processedas in Example 1 except that coating is at 220 mg. silver per square footand 425 mg. gelatin per square foot, and development is for 3.75 minutesin Kodak developer D-85. The results are shown in Table II. In thistable, Dye refers to the halogen or electron acceptor.

at 65 C., and then adding 4.0 mg. potassium chloroof water are thenstirred in, and the emulsion cooled. During both additions of the silvernitrate and sodium chloride (i.e., to form both the core and the shell),the two solutions are added at approximately constant rates. Sufficientsilver chloride is formed in the shell to give a ratio of 4 moles ofshell silver chloride to 1 mole of core silver chloride. The resultingcovered grain emulsion is melted, the gelatin content increased to 160g. per mole of silver chloride, and water added to 4000 grams per moleof silver chloride. The emulsion obtained is divided into severalportions and electron acceptors, halogen acceptors, potassium iodide andpotassium bromide are added as indicated in Table III. The severalemulsions are coated on a cellulose acetate film support at a coatingrate of 180 mg. silver per square foot and 400 mg. gelatin per squarefoot, chill set and dried. The coatings are exposed and processed asdescribed in Example 1, except that development is for seconds in KodakD-72 developer diluted with an equal volume of water. The results areshown in Table III.

TABLE II Relative Dmnxin Dmin. in Halogen or electron Order of KI g./KBr g./ clear unexposed exposed acceptor g./mole Ag addition mole molespeed area area 10.0). 0. 21 1. 34 0. 93 I(1.0) Dye first s. 70 0.10 0.04 10.0 .0 8.7 1. 34 0. 27 0 363 1.17 0.07 22.0 0. 54 0.13 1.3 1. 420.21 1.0) 00. 0 0. 7s 0. 22 0. 70 1. 32 0.67 111(05) 0 240 0. 74 0.08111(05) 0. 03 1. 1. 0 Hum) 13s 1. 20 0. 0s III(O.5) 331 1. 48 0.8 mm. 0.05 1. 51 1. 0 III(0.5) e0. 0 1. 40 0. 00 IV(0.5) 5s 1. 50 0. 03 0. 7950. 90 0.06 1v 0.5 do 120 1.66 0.15

Example 3 illustrates the practice of the invention using a directpositive emulsion of the type containing grains TABLE 111 comprising acentral core of a water insoluble silver salt Halogen or containingcenters which promote the deposition of photoelectrgn accep or Dru inDm; .in lyt 1c silver and an outer shell covering the core com (gilmoleKlg KBT Relative unextgosed expgsed prising a fogged water insolublesliver salt that develops to Ag) mole Ag mole Ag clear speed area areasilver without exposure. 71 0 1. 78 0. 06 1.66 056 Example 3 1.58 0. 00A gelatino silver chloride emulsion is prepared by {22 2:22simultaneously adding, over a period of about 20 minutes, 1000 ml. of a4 molar silver nitrate aqueous solution and 39 03 1000 ml. of a 4 molarsodium chloride aqueous solution, H to a well-stirred aqueous solutionof 1000 ml. of 0.01 molar sodium chloride at C. containing 40 grams of1%: 1 1. $8 gelatin. Thereafter, 5000 ml. of water contalning 280 5grams of gelatin is added and the emulsion cooled. One- 4% [1).83 eighthof the resulting gelatino silver chloride emulsion 1 (containing 0.5mole percent silver chloride) is melted 12i 811% at 40 C., mg. of thewater-soluble iridium salt, potassium chloroiridite, dissolved in waterare added, and 32 g- 2 the emulsion is heated to 70 C. This preparedemulsion constitutes the silver chloride core containing physical i-igg? discontinuities that accept (or trap) electrons over which 22 iscoated a shell of silver chloride. The shell of silver $3 8 3 chlorideis formed by adding to the core emulsion 500 ml. 25 25 of 4 molar silvernitrate aqueous solution and 500 ml. of 4 :32 molar sodium chlorideaqueous solution simultaneously Q78 0,02 over a period of 20 minutes.This silver chloride of this 8-83 shell is reduction and gold fogged byadding 0.2 mg. 2; g

thiourea dioxide per mole of silver, heating for 60 minutes Examples 4and 5 show the practice of this invention using a uniformly foggedsilver chlorobromide emulsion, the halide of the silver chlorobromidebeing 90 mole percent chloride.

Example 4 An emulsion is prepared as described in Example 1, except thata sufficient quantity of potassium bromide is added during theprecipitation to form a silver chlorobromide consisting of 90 molepercent chloride. The emulsion is divided into several portions and ahalogen acceptor and an electron acceptor are added to various portionsboth with and without the addition of potassium bromide, as indicated inTable IV. The emulsions are coated on a cellulose acetate film supportat the rate of 100 mg. silver per square foot and 370 mg. of gelatin persquare foot. The coatings are chill set, dried, exposed and processed asdescribed in Example 1, except that development is conducted for oneminute. The results are shown in Table IV.

TABLE IV Dwain D ll1 Halogen or electron KBr g./ Relative unexposedexposed acceptor (gJmole Ag) mole Ag clear speed areas areas Example 5 Agelatin silver chlorobromide emulsion containing 90 mole percentchloride is prepared in a manner similar to the emulsion of Example 3,sufficient sodium bromide being added, along with sodium chloride, toform core and shell emulsions containing 90 mole percent chloride. Theemulsion is split into several portions which are tested with anelectron acceptor and a halogen acceptor, both with and without theaddition of potassium bromide as indicated in Table V. The emulsions arecoated and tested exactly as described in Example 4. The followingresults are obtained.

TABLE V Dim. in unn. n Halogen or electron KBr g./ Relative unexposedexposed acceptor (gJmole Ag) mole Ag clear speed areas areas I(0.5)0.55 1. 50 1. 4 I(0.5) 4. 38 7. 6 1. 34 O. 04 IIl'(0.5) 100.0 1. 42 0.07III(0.5) 4. 38 263. 0 1. 17 0. 07

Electron acceptors which are not dyes can be used in the practice ofthis invention. This is illustrated in Example 6.

Example 6 An emulsion prepared as described in Example 2 is divided intoseveral portions and Compound I, which is an electron acceptor but not adye, is added to various portions of the emulsion with and without theaddition of potassium bromide. The emulsions are coated, exposed andprocessed as described in Example 2 with the following results.

Dye IV-3-ethyl-5-[l-(4-sulfobutyl)-4( lH)-pyridylidene]rhodanine sodiumsalt (a halogen acceptor) Dye V2- [2- 3 ,5 dimethyl- 1phenyl-4-pyrazolyl vinyl] 1 ,3-diphenylimidazo [4,5 b] quinoxaliniumiodide (an electron acceptor) DyeVI-3-[(l,3-diethyl-2(1H)-imidazo[4,5-b]quinoxa linylidene)ethylidene]-2H-pyrido[ l,2-a]pyrimidine- 2,4-(3H)-dione (an electron acceptor) DyeVII-3-ethyl-2-[ (2-methyl-5-oxo-3phenyl-3-isoxazolin-4-yl vinyl]o-nitrobenzothiazolium methylsulfate (an electron acceptor) Dye VIII1,1,3,3,3,3-hexamethyl-5,5'-dinitroindocarbocyanine p-toluenesulfonate(an electron acceptor) Dye IX5 ,5'-dich1oro-3,3diethyl-6,6-dinitrothiacarbocyanine iodide (an electronacceptor) Dye X3 (3-ethyl-6-nitro-2-benzothiazolinylidene)- ethylidene]-2H-pyrido[ 1,2-a] pyrimidine-2,4(3H) dione (an electron acceptor)Compound Il,l'-diethyl-2,2-cyanine chloride, dibrominated (i.e., thecarbon atom joining the two nuclei carries two bromine atoms) Resultssimilar to those in Example 1 are obtained with a silver chlorideemulsion foggedwith high intensity light or with reducing agent alone,e.g., stannous chloride. Good results are also obtained whencombinations of halogen acceptor with electron acceptor are used on thesurface of the silver halide grains.

The invention has been described in detail with particular reference topreferred embodiments thereof, but, it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention described hereinabove and in the appended claims.

I claim:

1. In a direct positive photographic emulsion comprising silver halidegrains, the halide of said silver halide being at least mol percentchloride, at least the outer shell of said grains being substantiallyuniformly fogged, said grains containing on the surface thereof acommand selected from the group consisting of:

(1) an electron acceptor having an anodic polarographic half-wavepotential and a cathodic polarographic half-wave potential, which, whenadded together, gives a positive sum, and

(2) a halogen accepter having an anodic polarographic half-wavepotential less than 0:85 and a cathodic polarographic half-wavepotential which is more negative than l.0;

the improvement which comprises a sufficient quantity of halide on thesurface of said silver halide grains to effectively increase the speedof said silver halide, said halide being selected from the groupconsisting of bromide and a mixture of bromide and iodide, and saidquantity of halide being in addition to any halide present in saidgrains as mixed silver halide.

2. In a direct positive photographic emulsion comprising silver halidegrains, the halide of said silver halide being at least 50 mol percentchloride, at least the outer shell of said grains being substantiallyuniformly fogged, said grains containing 0n the surface thereof acompound selected from the group consisting of: TABLE VI Relative Dmax.in Dmin. in

KI g./ KBr g./ clear unexposed exposed Electron acceptor (g./mole) molemole speed area area Cpd. no.5) 0.19 1. e2 1, 2 Cpd. I(0.5) 4. 38 0.070, 03 Cpd. no.5 4. 38 18.0 1.08 0. 04

The halogen acceptors and electron acceptors employed in the aboveexamples are identified below:

Dye Il,3-diethyl-lmethyl-Z-phenylimidazo[4,5-b1-quinoxalino-3'-indolocarbocyanine iodide (an electron acceptor) DyelIPinacryptol yellow (an electron acceptor) Dye IlI3-carboxymethyl-5-[(3-methyl-2 (3 thiazolinyldene)isopropylidene]rhodanine (a halogenacceptor) 19 the improvement which comprises a sufficient quantity ofhalide on the surface of said silver halide grains to effectivelyincrease the speed of said silver halide, said halide being selectedfrom the group consisting of bromide and a mixture of bromide andiodide, and said quantity of halide being in addition to any halidepresent in said grains as mixed silver halide.

3. A fogged direct positive photographic emulsion as defined in claim 2wherein said halide comprises a quantity of bromide of from about .01 toabout 0.2 mole per mol of silver.

4. A fogged direct positive photographic emulsion as defined in claim 2wherein said halide is a mixture of bromide and iodide, the quantity ofbromide is from about .01 to about 0.2 mole per mol of silver and thequantity of iodide is from about .002 to about .03 mole per mol ofsilver.

5. A fogged direct positive photographic emulsion as defined in claim 2wherein said halide is a mixture of bromide and iodide, the quantity ofbromide is from about .04 to about .09 mole per mol of silver and thequantity of iodide is from about .003 to about .012 mole per mol ofsilver.

6. A fogged direct positive photographic emulsion as defined in claim 4wherein said silver halide grains are chemically fogged and the halideof said silver halide is at least 80 mol percent chloride.

7. A fogged direct positive photographic emulsion as defined in claim 4wherein said silver halide grains are fogged with the combination of areducing agent and a compound of a metal more electropositive thansilver; and, the halide of said silver halide is at least 80 mol percentchloride.

8. A fogged direct positive photographic emulsion as defined in claim 4wherein said electron acceptor is a photographic sensitizing dye whichspectrally sensitizes the emulsion so that the ratio of relative minusblue speed to relative blue speed is greater than 7.

9. A fogged direct positive photographic emulsion as defined in claim 4wherein said compound is an electron acceptor, and is selected from thegrou consisting of a 2-aromatically substituted indole dye; animidazo[4,5-b] quinoxaline dye; a pyrazolyl dye; a pyrrolo[2,3-b]pyridodye; a nitro-substituted dye; and, the reaction product of a cyanine dyewith a halogenating agent.

10. A fogged direct positive photographic emulsion as defined in claim 4wherein said compound is a halogen acceptor, and has the followingformula:

wherein n represents a value from to 2; L represents a methine linkage;B represents the atoms required to complete a basic nitrogen containingheterocyclic nucleus; and, A represents the atoms required to completean acidic heterocyclic nucleus.

11. A fogged direct positive photographic emulsion comprising silverhalide grains substantially uniformly fogged with a combination ofthiourea dioxide and potassium chloroaurate, the halide of said silverhalide being at least 80 mol percent chloride, said grains containing011 the surface thereof, as electron acceptor, 1,3-diethyl- 1' methyl 2phenylimidazo[4,5-b]quinoxalino 3 indolocarbocyanine iodide; and, thesurface of said grains having thereon from about .04 to about .09 molebromide per mol of silver and from about .003 to about .012 mole iodideper mol of silver, said bromide and iodide being in addition to anybromide or iodide present in said grains as mixed silver halide.

12. A fogged direct positive photographic emulsion comprising silverhalide grains substantially uniformly fogged with a combination ofthiourea dioxide and potassium chloroaurate, the halide of said silverhalide being at least 80 mol percent chloride, said grains containing onthe surface thereof, as halogen acceptor, 3-carboxymethyl 5 [(3 methyl2(3) thiazolinylidene)isopropylidene]rhodanine; and, the surface of saidgrains having thereon from about .04 to about .09 mole bromide per molof silver and from about .003 to about .012 mole iodide per mol ofsilver, said bromide and iodide being in addition to any bromide oriodide present in said grains as mixed silver halide.

13. In the process for increasing the speed of a substantially uniformlyfogged, direct positive emulsion comprising silver halide grains, thehalide of said silver halide being at least 50 mol percent chloride, atleast the outer shell of said grains being substantially uniformlyfogged, which process includes contacting the surface of said grainswith a compound selected from the group consisting of:

(1) an electron acceptor having an anodic polarographic half-wavepotential and a cathodic polarographic half-wave potential, which, whenadded together, gives a positive sum, and

(2) a halogen acceptor having an anodic polarographic half-wavepotential less than 0.85 and a cathodic polarographic half-wavepotential which is more negative than 1.0;

the improvement which comprises contacting the surface of said grainswith a sufficient quantity of Water-soluble halide salt to eifectivelyincrease the speed of said silver halide grains, said halide salt beingselected from the group consisting of a bromide salt and a mixture of abromide salt and an iodide salt.

14. In the process for increasing the speed of a substantially uniformlyfogged, direct positive emulsion comprising silver halide grains, thehalide of said silver halide being at least 50 mol percent chloride, atleast the outer shell of said grains being substantially uniformlyfogged, which process includes contacting the surface of said grainswith a compound selected from the group consisting of:

(1) an electron acceptor having an anodic polarographic half-Wavepotential and a cathodic polarographic half-wave potential, which, whenadded together, gives a positive sum, and

(2) a merocyanine dye halogen acceptor having an anodic polarographichalf-wave potential less than 0.85 and a cathodic polarographichalf-wave potential which is more negative than 1.0;

the improvement which comprises contacting the surface of said grainswith a sufficient quantity of water-soluble halide salt to effectivelyincrease the speed of said silver halide grains, said halide salt beingselected from the group consisting of a bromide salt and a mixture of abromide salt and an iodide salt.

15. The process as defined in claim 14 wherein said compound is added tothe emulsion prior to the addi tion of said halide salt.

16. The process as defined in claim 14 wherein said halide saltcomprises a quantity of a bromide salt of from about .01 to about 0.2mole per mol of silver.

17. The process as defined in claim 15 wherein said halide saltcomprises a mixture of a bromide salt and an iodide salt, theconcentration of said bromide salt being from about .01 to about 0.2mole per mol of silver and the concentration of said iodide salt beingfrom about .002 to about .03 mole per mol of silver.

18. The process as defined in claim 15 wherein said halide saltcomprises a mixture of a bromide salt and an iodide salt, the quantityof bromide salt being from about .04 to about .09 mole per mol of silverand said quantity of iodide salt being from about .003 to about .012mole per mol of silver.

19. The process as defined in claim 17 wherein said compound is anelectron acceptor, and said electron acceptor is a sensitizing dye whichspectrally sensitizes the emulsion so that the ratio of relative minusblue speed to relative blue speed is greater than 7.

20. The process as defined in claim 17 wherein said compound is anelectron acceptor, and said electron ac ceptor is selected from thegroup consisting of a 2-aromatically substituted indole dye; an imidazo[4,5-b]quinoxaline dye; a pyrazolyl dye; a pyrrolo[2,3-b]pyrido dye; anitro-substituted dye; and, the reaction product of a cyanine dye with ahalogenating agent.

21. The process as defined in claim 17 wherein said compound is ahalogen acceptor, and said halogen acceptor has the formula:

wherein n represents a value from to 2; L represents a methine linkage;B represents the atoms required to complete a basic nitrogen containingheterocyclic nucleus; and, A represents the atoms required to completean acidic heterocyclic nucleus.

22. The process for increasing the speed of a direct positive emulsioncomprising silver halide grains substantially uniformly fogged with thecombination of thiourea dioxide and potassium chloroaurate, the halideof said silver halide being at least 80 mol percent chloride, whichcomprises adding to said emulsion, as electron acceptor,1,3-diethyl-1-methyl-2-phenylimidazo[4,5 b]quinoxalino-3-indolocarbocyanine iodide, followed by the addition tosaid emulsion of from about .04 to about .09 mole potassium bromide permol of silver and from about .003 to about .012 mole potassium iodideper mol of silver.

23. The process for increasing the speed of a direct positive emulsioncomprising silver halide grains substantially uniformly fogged with acombination of thiourea dioxide and potassium chloroaurate, the halideof said silver halide being at least 80 mol percent chloride, whichcomprises adding to said emulsion, as halogen acceptor, 3-carboxymethyl5-[(3 methyl-2(3)-thiazolinylidene)isopropylidene]rhodanine, followed bythe addition to said emulsion of from about .04 to about .09 molepotassium bromide per mol of silver, and from about .003 to about .012mole potassium iodide per mol of silver.

24. The process as defined in claim 17 wherein said silver halide grainsare chemically fogged and said silver halide is composed of at least 80mol percent chloride.

25. The process as defined in claim 17 wherein said 22 silver halidegrains are fogged with a combination of a reducing agent and a compoundof a metal more electropositive than silver, and said silver halidegrains contain at least mol percent chloride.

26. A direct positive, photographic emulsion in accordance with claim 2which comprises fogged silver halide grains, said grains being such thata test portion thereof, when coated as a photographic silver halideemulsion on a support to give a maximum density of at least about 1 uponprocessing for 6 minutes at about 68 F. in Kodak DK-SO developer, has amaximum density which is at least about 30% greater than the maximumdensity of an identical coated test portion which is processed for 6minutes at about 68 F. in Kodak DK50 developer after being bleached forabout 10 minutes at about 68 F. in a bleach composition of:

Potassium cyanide50 mg. Acetic acid (glacial)3.47 cc. Sodiumacetate-11.49 g. Potassium bromide119 mg. Water to 1 liter 27. A directpositive, photographic emulsion in accordance with claim 2 whichcomprises fogged silver halide grains, at least by weight, of saidgrains having a size which is within about 40% of the average grainsize.

28. A photographic element comprising a support having coated thereon adirect positive photographic emulsion as defined in claim 2.

29. A photographic element comprising a support having coated thereon adirect positive photographic emulsion as defined in claim 4.

References Cited UNITED STATES PATENTS 2,592,250 4/1952 Dovey et al.96-94 X 3,314,796 3/1962 Gotze et al. 96--101 3,364,026 1/1968 Rees96101 X 3,367,778 2/ 1968 Berrimon 96-64 NORMAN G. TORCHIN, PrimaryExaminer R. E. FIGHTER, Assistant Examiner US. Cl. X.R. 96101, 107

